I have a 99 626 4cyl that is giving me fits. Symptoms include: not starting for days, (cranks fine but no fire) then randomly starting fine a few times. While it has been in a no-start condition, I have verfied fuel and spark, as well as monitored live data from things like the cam position sensor and crank sensor, as well as the coolant temp sensor. I was thinking bad ground, so I hooked jumpers from the block to the battery and still no good. Did a compression test and its fine - a little low, but at least even across all four cylinders.
Other symptoms (maybe not related) I have no dash lights or dome lights despite having good fuses.
This sounds very much like when the alternator on my J2000 dies all those many years ago (it was in '89 and the car was 8 years old). The electrical system went kind of nuts while you were driving down the road, yet when I'd get to the dealership....they couldn't get it to repeat. When I mentioned that the dashboard lights were randomly flashing on and off (all of them, not just 1 or 2) they decided to take the alternator apart.
HOWEVER, I really don't think that is your problem nearly as much as I believe it is the main relay. When a main relay goes, it can be sort of intermittent at first....then it dies/sticks. And the car won't start with jumping when the main relay has died.
Would the main relay let it crank normally but not fire?
my '96 probe GT was doing this recently. i observed that it was occuring only when the temp was cold and the humidity was high. if i shut the engine off for the night while it was raining or very foggy, it was almost guaranteed to not start in the morning.
turns out something went wrong in the distributor cap, in the internal conductor between the coil output and the rotor button. resistance went way up for some reason, which actually melted a hole in the cap and would allow moisture inside the distributor. new distributor cap fixed it. one of these days i'll post pix of the old cap, as it was a pretty strange fail.
